The present invention relates to a circuit and especially but not exclusively to a circuit for use in controlling one or more fans with reference to temperature, and for detecting the failure or inadequacy of the fan or temperature sensors. WO 96/09688 discloses a circuit for controlling the driving voltage of a fan in electrical equipment in response to an input voltage from a thermistor. Control of the voltage with temperature is desirable to in crease the operating speed, and hence cooling effect, of a fan if the temperature of the area to be cooled rises.
The circuit of WO 96/09688 uses the commutated signal at the input of the fan to provide detection pulses allowing the speed of rotation of the fan to be determined. The detection pulses are converted to provide a signal which has, for each control pulse, a retriggerable pulse with a predetermined length, and a no-pulse period. The length of the no-pulse period increases as the difference between the actual fan speed and the desired fan speed (as determined by an input control voltage) increases. The signal is used for influencing the charge state of a storage component by asymmetrically charging and discharging the storage component with the alternate pulse and no-pulse period, so that discharging occurs considerably more quickly than charging. The storage component (a capacitor) is connected to the ground reference point of the circuit. The change in the charge state of the storage component provides a voltage to the base of a transistor which provides a driving voltage for the fan. The collector of the transistor is connected to the ground reference point of the circuit and the emitter is connected to the fan to provide a driving voltage.
The known circuit is illustrated schematically in FIG. 1, and is described further, with reference to FIG. 1, in the specific description of the preferred embodiments.
According to a first aspect of the present invention there is provided a control circuit for controlling a fan in which a commutated signal indicative of the fan speed is processed with reference to a reference signal indicative of a desired fan speed in order to give a control signal, wherein the control signal is processed via a charge/discharge circuit to generate a control voltage which is used to control the operating speed of the fan, and wherein there is provided means to monitor said control voltage to detect inadequate fan functioning.
Preferably the means to monitor the control voltage includes a comparator allowing said control voltage to be compared to a reference voltage, and wherein inadequate fan function is indicated either by said control voltage exceeding, or by said control voltage falling below, said reference voltage.
Preferably, a control signal is provided by a temperature sensitive device. Preferably, the temperature sensitive device is a thermistor.
Preferably, there are provided a number of temperature sensitive devices connected to the control circuit such that the output of the temperature sensitive device which corresponds to the temperature requiring the highest fan speed is utilised to provide the control signal. There may be provided a number of fans, each with a corresponding control circuit. Each temperature sensitive device may provide a control signal for one or more of the fans.
Where a temperature sensitive device provides a control signal for more than one fan, there is preferably included means to enable the temperature sensitive device to provide a control signal which is not dependent upon the number of fans. Said means may be an emitter follower circuit.
Preferably there is provided means to set a maximum fan speed. The means to set a maximum fan speed may comprise a means to provide a maximum fan speed signal, and means to utilise only one of:
a) the maximum fan speed signal; and
b) a variable signal provided to affect fan speed;
to control the fan speed, wherein the signal which is utilised is that which requires the lower fan speed.
Preferably there is provided means to set a minimum fan speed. The means to set a minimum fan speed may comprise means to provide a minimum fan speed signal,
and means to utilise only one of:
a) the minimum fan speed signal; and
b) a variable signal provided to affect fan speed;
to control the fan speed, wherein the signal which is utilised is that which requires the higher fan speed.
Preferably, there is provided means to provide a warning signal in response to a detected temperature exceeding a given predetermined temperature.
Preferably, there is provided means to provide a warning signal, and/or turn off one or more fans, in response to a detected temperature falling below a given predetermined temperature.
Preferably, there is provided means to provide a control signal which results in all fans being turned off. Such control signal may be provided as a result of an input to the circuit.